The present invention relates, in general, to semiconductor device fabrication processes and, more particularly, to processes for achieving consistent device characteristics on a semiconductor substrate.
The fabrication of semiconductor devices often includes forming doped regions near trenches on a semiconductor wafer. For example, in a dynamic random access memory (DRAM) fabrication process, deep trenches are formed in a semiconductor wafer. A dopant containing oxide layer is deposited on the semiconductor wafer. The oxide layer covers the front surface of the wafer as well as the sidewalls and bottoms of the trenches. In a spin-on process, photoresist is applied to the wafer, covering the dopant containing oxide layer and filling the trenches. A dry etching process removes the photoresist over the wafer front surface. The etching process also removes upper portions of the photoresist filling the trenches, thereby creating recesses in the trenches. The recesses expose the dopant containing oxide layer on the upper portions of the sidewalls in the trenches, which is subsequently removed in an oxide etching process. The photoresist in the lower portions of the trenches is then etched away. The wafer goes through an annealing process, in which the dopant in the oxide layer diffuses into the semiconductor substrate, forming doped regions along the lower portions of the trenches. The upper boundaries of the doped regions in the trenches are recessed from the front surface of the wafer.
Because of process variation, the thickness of the photoresist is usually not uniform over the wafer. The dry etch process translates the inhomogeneous photoresist thickness into inhomogeneous recesses in the trenches. Consequently, the recesses of the upper boundaries of the doped regions vary from trenches to trenches. In other words, the recess depths of the doped regions in different trenches are different from each other. The inhomogeneity in the recess depths may adversely affect the characteristics, performance, and reliability of the fabricated DRAM. It may also affect the yield of the fabrication process.
Accordingly, it would be advantageous to have a method or a process for controlling the recess depth in trench to achieve substantially homogenized device parameters throughout a semiconductor die or a semiconductor wafer. It is desirable for the method to be simple and reliable. It would be of further advantage for the process to be compatible with conventional fabrication processes.
A general object of the present invention is to provide a reliable semiconductor device fabrication process to homogenize semiconductor device parameters and achieve substantially uniform device characteristics. More particularly, an object of the present invention is to control the photoresist recess depth in trenches in a semiconductor wafer, thereby achieving a substantially uniform recess depth in trenches throughout the wafer. It is a further object of the present invention for the fabrication process to be simple reliable, and compatible with other semiconductor device fabrication processes.
These and other objects of the present invention are achieved by planarizing the photoresist disposed over a semiconductor wafer, thereby controlling the thickness of the photoresist over the wafer. For example, in a fabrication process in accordance with the present invention, photoresist is applied over a semiconductor wafer with trenches formed therein. The photoresist covers the front surface of the semiconductor wafer and fills the trenches. The disposed photoresist is planarized in a chemical mechanical polishing process to achieve a substantially uniform thickness throughout the wafer. After the planarization, an anisotropic etching process such as, for example, a reactive ion etching process partially removes the photoresist in the trenches, thereby creating recesses in the trenches. Because the thickness of the photoresist is substantially uniform throughout the wafer before the etching process, the depths of the recesses in different trenches in the wafer are substantially equal to each other. Therefore, a substantially uniform recess depth throughout the wafer is achieved. The uniform recess depth significantly facilitates in ensuring that the semiconductor devices fabricated on the wafer have consistent characteristics and performances.